1. Field of the Invention
The present invention relates to a heat radiating type liquid level sensing system and a method of detecting a liquid level on the basis of change in resistance of a resistive level sensor immersed into a liquid, and more specifically to a heat radiating type liquid level sensing system and the method therefor suitable for use in sensing fuel level in a fuel tank of an automotive vehicle.
2. Description of the Prior Art
In general, the resistance of substance increases with increasing substance temperature, as shown in FIG. 1A. Therefore, when a resistive sensor rod Ro heated by a constant current generator I is partially immersed in liquid, since the immersed portion of the sensor rod is cooled by the liquid, the total resistance of the sensor rod Ro decreases with increasing immersion depth of the sensor rod Ro, that is, with increasing liquid level, as shown in FIG. 1B.
However, the level sensor of this type is of course subjected to a severe influence of the ambient temperature, in addition to the immersion depth of the level sensor rod. To overcome this problem, conventionally a temperature correcting resistor r.sub.t is additionally provided in the sensing circuit in such a way that the resistance of the correcting resistor r.sub.t changes only according to the ambient temperature, without being heated, as shown in FIG. 2A. In FIG. 2A, the prior-art level sensor circuit is composed of the level sensor rod Ro, the temperature correcting resistor r.sub.t, a constant current generator I, a buffer amplifier op1, an operational amplifier op2, and a feedback resistor r.sub.f in order to reduce the influence of the ambient temperature.
In the prior-art liquid level sensor of this type, however, since the physical parameters and the liquid volume change (expands or contracts) according to ambient temperature and therefore the detected liquid level is still subjected to the influence of ambient temperature, there exists a problem in that it is still difficult to accurately detect liquid level.